Flexible packing material in sheet or web form

ABSTRACT

Flexible, creased material in sheet or web form for use in the manufacture of liquid-tight packing containers of good dimensional rigidity comprises one or more base layers laminated to one another. The base layer is a mineral-filled thermoplastics material comprising between 50 and 80% calculated on the total weight of the mixture, of an inorganic, particulate mineral filler. The thermoplastics material is chosen among propylene-based polymers with a melt index of between 0.5 and 5 according to ASTM (2.16 kg, 230° C.) and the base layer or base layers are provided with a pattern of crease lines which have been formed by plastic deformation in connection with or immediately after the extrusion of the base layer or base layers.

FIELD OF THE INVENTION

This invention relates to a flexible, creased material and moreparticularly, a flexible, creased material in sheet or web form for usein fabricating liquid-tight packing containers.

BACKGROUND OF THE INVENTION

In packing technology, packages of a non-returnable character which aremanufactured from a material comprising a base layer of paper orcardboard with outer and inner coatings of thermoplastics have been usedfor a long time. The material in these so called non-returnable packagesis often also provided with further layers of other material, e.g.Al-foil or plastic coatings other than those mentioned here.

The composition of the packing material is selected to create the bestpossible protection for the product which is to be packed. At the sametime the composition is selected to give the required mechanicalstrength and durability to enable it to withstand such outer externalstresses as the package is subjected to in normal handling. To achievethe necessary mechanical rigidity, which on the one hand givesmechanical protection to the product and on the other hand makes itpossible for the package to be dimensionally rigid enough to allow it tobe handled and manually gripped without difficulty, the material inthese packages is frequently provided with a relatively thick base layerof paper or cardboard. Such a material, however, is not liquid or gastight and the rigidity of the material is quickly lost when it issubjected to moisture. To impart the required liquid-tightness to thematerial the base layer is provided therefor, frequently on both sides,with a coating of plastic material, and if the plastic material isthermoplastics the coating may also be used for sealing the plasticcoatings to one another by so-called heat-sealing. In this mannerpackages can be sealed and made lastingly permanent in their intendedshape by heat-sealing together thermoplastic-coated, overlappingmaterial panels in liquid-tight and mechanically durable sealing joints.

Non-returnable packages of the type referred to here are manufactured inmost cases with the help of packing machines which, either from a web orfrom prefabricated blanks of a packing material, form, fill and sealfinished packages at a high rate of production. Packages aremanufactured, e.g. from a web by joining together the longitudinal edgesof the web in an overlap joint so as to form a tube which issubsequently filled with the actual contents, and through repeatedflattening and sealing of the tube, at right angles to the longitudinalaxis of tube, is divided into closed packages. Finally, the packagingunits are severed from one another by means of cuts in the transversesealing zones and are given the desired geometrical shape, usually aparallellepiped, by further folding and sealing.

During the manufacture of packages in the manner described above thelaminated material is subjected to stresses which become particularlylarge during folding of the material. During folding, and as a result ofthe relatively great material thickness of the base layer, the oneplastic coating is subjected to a strong stretching force while at thesame time, the other plastic coating is compressed to a correspondingdegree along the folding line. Due to the great extensibility of theplastic coatings, the folding and resultant stretching of the plasticmaterial only rarely leads to breaks or other damages causing leakage inthe plastic coating. However, the problem is aggravated if the materialalso comprises an Al-foil layer which, compared with the plasticcoatings, is not as stretchable and consequently tends to crack when thematerial is folded.

Even if a single 180° folding of the material normally does not have anyserious consequences, considerable difficulties arise when the materialis to be folded along two crossing crease lines. This is often the casein external sealing areas always occurring on this type of packages,irrespectively of whether they are manufactured from a web or fromprefabricated blanks. The sealings generally are carried out by heatingand melting the plastic coating facing towards the inside of the packagealong the edge zones which are to be sealed to one another. Thereafterthe heated plastic coatings are pressed against one another to form asealing fin held together through surface fusion on the outside of thepackage. Such a fin comprises double material layers, and to ensure thatit does not form an obstacle, the fin frequently is folded down to lieflat against the outside of the package, which means that one of thematerial layers of the sealing fin is folded over a 180° and that thepackage wall in the region of the folded-down fin comprises threematerial layers, that is to say, has a threefold material thickness.Such a sealing fin often runs along one or more side faces of thepackage, and since these side faces in the forming of, for example,parallellepipedic packages are subjected to a 180° folding along acrease line at right angles to the sealing fin, the material thicknessin certain regions of the package will be six times the laminatethickness. At this in the area of the 180° folding that is transverse tothe sealing region the material layers located outermost will besubjected to very strong tensile stresses with accompanying extensionsand increased risks of crack formation in the material. These tensilestresses frequently are so great that cracks occur not only in theAl-foil included in the material, but also in the thermoplasticcoatings. Cracks in the thermoplastic coating can result in leakage ofthe packed contents which can readily be absorbed by, and impair therigidity of, the base layer of the material.

OBJECTS AND SUMMARY OF THE INVENTION

Since conventional packing material based on paper or cardboard isobviously associated with serious disadvantages related essentially tothe use of a fibrous layer which of necessity has to be relatively thickin order to impart mechanical rigidity to the material, it has been anobject for a long time to find a new packing material for themanufacture of liquid-tight packing containers which does not includefibrous material but which nonetheless possesses good dimensionalrigidity. It is an object of the present invention thus to providedirections concerning such packing material free of paper or cardboard.

It is a further object of the invention to provide a packing materialwhich with the help of modern, high-capacity packing machines can beconverted readily to liquid-tight, dimensionally rigid packingcontainers without the risk of crack formations when the material isfolded during the manufacture of the containers.

These along with further objects are achieved in accordance with thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following in more detail withspecial reference to the attached drawings, wherein like members bearlike reference numerals and wherein:

FIG. 1 is a plan view of a packing material in web form in accordancewith one embodiment of present invention;

FIG. 2 is an enlarged cross-sectional view of the material in FIG. 1along the sectional line II--II;

FIG. 3 is a schematic illustration of an arrangement for the manufactureof the packing material;

FIG. 4 is a plan view of a packing material in web form in accordancewith a second embodiment of the present invention;

FIG. 5a is an enlarged cross-sectional view of the material in FIG. 4along the line V--V;

FIG. 5b is a cross-sectional view similar to that in FIG. 5a of apacking material in accordance with a further embodiment of the presentinvention, and

FIG. 6 is a schematic illustration of an arrangement for the manufactureof the packing material in FIGS. 4 and 5a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a whole package length L of a material in web formaccording to the invention which has been given the general referencedesignation 1. From the web 1 are manufactured packing containers, aspointed out earlier, where both longitudinal edge zones 2 of the web 1are joined to one another in an overlap joint so as to form a tube whichsubsequently is filled with the actual contents. The filled tube isdivided thereafter into individual container units by means of repeatedflattening and sealing of the tube along narrow transverse sealing zones3 at right angles to the longitudinal axis of the tube. Finally thecontainer units are separated from one another by cuts in the transversesealing zones 3 and are given the desired shape, e.g. parallellepipedic,through a further forming and sealing operation.

The material in accordance with this embodiment of the presentinvention, as is evident from FIG. 2, comprises a base layer 4 ofmineral-filled propylene-based polymer with a melt index of between 0.5and 5 according to ASTM (2.16 kg, 230° C.). The quantity of filler inthe polymer may vary between 50 and 80% of the total weight of themixture, but preferably is within 65-75% by weight, which has been foundto produce optimum properties of the material with regard to rigidityand modulus of elasticity.

The propylene-based polymer may be a polypropylene homopolymer with amelt index of below 1 according to ASTM (2.16 kg, 230°), but preferablythe propylene-based polymer is chosen among propylene-ethylenecopolymers with a melt index within the above specified range of between0.5 and 5 since those copolymers have been able to withstand folding andbending operations without cracking even at low temperatures whichnormally occur during the conversion of the packing material into foldpacking containers and the subsequent filling of the fold packingcontainers with liquid food, such as milk.

The choice of filler is not critical in accordance with the invention,but, practically speaking, the whole range of known mineral fillers maybe used, e.g. mica, talc, calcium salts such as calcium sulphate orcalcium carbonate etc. A base layer containing 65% by weight of calciumcarbonate in particle form with a grain size of under 10 μm, however,has been found to be the material combination that functions well inpractice and that makes possible the manufacture of liquid-tight packingcontainers with the desired good dimensional rigidity. In practice, thematerial thickness d of the base layer may vary between 100 and 400 μm,but preferably amounts to 300 μm.

To facilitate the conversion of the web 1 to packing containers the baselayer 4 has been provided with an arbitrary pattern of crease lines 5and 6 for facilitating the folding, which respectively extend paralleland transversely to the longitudinal direction of the web. In theexample shown the crease lines 5 and 6 have been formed by plasticdeformation of the base layer 4 in connection with or immediately afterextrusion of said layer.

The material in web form 1 described above can be manufactured with thehelp of an arrangement of the type shown schematically in FIG. 3. Thearrangement comprises an extruder 7 with a suitably dimensioned nozzle 8and two feed-funnels 9 and 10 for charging the starting materialnecessary for the extrusion, that is to say granulated propylene-basedpolymer with a melt index of between 0.5 and 5 according to ASTM (2.16kg, 230°), such as a polypropylene homopolymer or a propylene-ethylenecopolymer, and mineral filler respectively. The thermoplastic masscontaining between 50% and 80%, 65%, by weight of filler is heated tosoftening or incipient melting (approx. 180°-300° C.), and is extrudedthrough the nozzle 8 to form a 100-400 μm, e.g. 300 μm, film 1'. Thefilm 1' is passed through the nip between cooperating, cooled pressurecylinders 11 and 12. One cylinder 11 is provided on its outer surfaceswith a pattern formed by raised surface or matrices which are pressedagainst the film 1' to deposit a complementary surface pattern on oneside of the film formed through plastic deformation to produce thecrease lines 5 and 6. The cooled film 1 provided with crease linessubsequently can be rolled up on a magazine roll 13.

FIG. 4 and FIG. 5a show to a whole package length L of a packingmaterial in web form in accordance with a second embodiment of thepresent invention which has been given the general reference designation14. From the material web 14 are manufactured filled, liquid-tightpacking containers, as described previously, wherein longitudinal edgezones 15 of the web 14 are joined to one another in an overlap joint soas to form a tube which subsequently is filled with the intendedcontents. The filled tube is separated thereafter into sealed containerunits by means of repeated flattening and sealing of the tube alongtransverse sealing zones 16 at right angles to the longitudinal axis oftube. Thereafter the container units are given the desired geometricalendform, e.g. parallellepipedic, by a further folding and sealingoperation.

As is evident from FIG. 5a the material according to this embodimentcomprises a base 17 consisting of base layers 17a and 17b laminated toone another which are manufactured by means of extrusion of a mixturecontaining a mineral-filled propylene-based polymer with a melt index ofbetween 0.5 and 5 according to ASTM (2.16 kg, 230° C.) and between 50%and 80%, preferably 65-70%, calculated on the total weight of themixture, of a particulate, inorganic mineral filler. The propylene-basedpolymer with a melt index within the above specified range may be apropylene homopolymer with a melt index of below 1 according to ASTM(2.16 kg, 230° C.), but for reasons described earlier is preferably apropylene-ethylene copolymer.

The filler used in the propylene-based polymer of the base layers 17aand 17b may be mica, talc, calcium salts such as calcium sulphate orcalcium carbonate etc. In the example shown it is assumed, however, thatone base layer 17a, that is the one facing towards the inside of theintended packing container, contains mica, whereas the other base layer17b contains calcium carbonate in particle form with a grain size ofunder 10 μm. The thickness of the respective base layers 17a and 17bincluded in the base 17 may vary between 50-200, but out of practicleconsiderations the thickness of the respective base layers is preferably100 μm.

FIG. 5b shows a cross section corresponding to that in FIG. 5a of apacking material in accordance with a further embodiment of theinvention. According to this further embodiment the material comprises abase 18 with outer base layers 18a and 18b of the same material as thebase layers 17a and 17b described above with reference to FIG. 5a, whichare joined to one another by an intermediate layer 18c of a foamed ordensity-reduced propylene-based polymer with a melt index of between 0.5and 5 according to ASTM (2.16 kg, 230° C.).

The total material thickness of the base 18 may vary, but out ofpractical considerations is usually approximately 300 μm, all the layersincluded in the base 18 preferably having the same mutual layerthickness, that is to say 100 μm.

To facilitate the folding of the web 14 in the manufacture of packingcontainers in the manner described above, the web 14, as is evident fromFIG. 4, has been provided with an arbitrary pattern of longitudinal andtransverse crease lines 19 and 20 respectively. These crease lines (onlythe longitudinal crease lines 19 whereof are shown in FIG. 5a) have beenformed by a plastic deformation of the one side or as in the exampleshown, of both sides of the base 17. In the same manner correspondingcrease lines, which for the sake of greater clarity have been given thesame reference designations as in FIG. 5a, have been formed in the base18 shown in FIG. 5b.

FIG. 6 shows schematically an arrangement for the manufacture of the web14 in accordance with the invention shown in FIG. 4 and FIG. 5a. Thearrangement comprises a coextruder of a known type having with asuitably dimensioned nozzle 23 comprising two slot-shaped openingsthrough which respective base layers 17a and 17b included in thematerial are coextruded from starting material necessary for therespective layers. The web 14 laminated from the co-extruded base layersis passed through the nip between two co-operating, coold pressurecylinders 21 and 22 while the web is soft. The outer surfaces of thecylinders 21, 22 are provided with raised portions or matrices of adesign which is such that when pressed against the web 14 while the web14 is passing between the two cylinders they produce a pattern of creaselines on both sides of the web by plastic deformation. The crease linesfacilitate folding of the web. The cooled web 14 provided with creaselines can subsequently be rolled up on a magazine roll which is notshown. In a similar manner the weblike packing material in accordancewith the further embodiment shown in FIG. 5b can be manufactured. Inthat case, the extruder includes a nozzle with three slot-shapedopenings so as to make possible a co-extrusion of the three layers18a-18c included in the base 18.

While this invention has been illustrated and described in accordancewith a preferred embodiment, it is recognized that variations andchanges may be made and equivalents employed herein without departingfrom the invention as set forth in the claims.

What is claimed is:
 1. A web for liquid tight packing containerscomprising:a first extruded base layer of mineral-filled thermoplasticmaterial having between about 50% and about 80%, based on the totalweight of the thermoplastic material, of an inorganic particulatemineral filler, said thermoplastic material being a propylene-basedpolymer having a melt index of between 0.5 and 5.0 according to ASTM(2.16 kg, 230° C.), said first base layer being provided with aplurality of crease lines in a surface of said first base layer, saidcrease lines being formed, through plastic deformation, concurrentlywith the extrusion of the first base layer or immediately thereafterwhile the first base layer is in a softened state; a second extrudedbase layer which is laminated to the first base layer, one of said firstand second base layers containing mica and the other base layerincluding calcium salt, whereby when the web is formed into a container,the base layer containing mica is on the interior of the container; andan intermediate layer of foamed propylene-based polymer positionedbetween said first and second base layers and joining said two baselayers together, said foamed propylene-based polymer of saidintermediate layer having a melt index of between 0.5 and 5.0 accordingto ASTM (2.16 kg. and 230° C.).
 2. The web according to claim 1, whereinsaid calcium salt is calcium sulphate.
 3. The web according to claim 1,wherein said calcium salt is calcium carbonate.
 4. The web according toclaim 1, wherein said first and second base layers are co-extruded. 5.The web according to claim 1, wherein the thickness of each of said baselayers is between about 50 μm and 200 μm and said base layers areflexible.
 6. The web according to claim 1, wherein said base layers andsaid intermediate layer are co-extruded.
 7. The web according to claim1, wherein said base layer has oppositely positioned side edges, some ofsaid crease lines extending from one side edge to the oppositelypositioned side edge of the base layer.